Nov 22, 2024

Michigan winters are not cold enough to reduce risk of volunteer potatoes

Potatoes are an important crop to Michigan’s economy so it might come as a surprise to some that not all potatoes are desirable. Volunteer potatoes are a unique type of weed: they are potatoes left in the soil after a potato crop is harvested.

Volunteer potatoes emerge in the spring and become weeds for corn, soybean and rotational crops. When winters were sufficiently cold in the past, volunteers were not an issue. The leftover potatoes would freeze in the soil. With warming winters, unwanted potato plants have become an issue almost every year.

Leftover tubers in the ground produce new daughter tubers and can continue to emerge years after the initial planting, which can be problematic. The re-emergence not only poses a contamination risk and competes with other field crops for resources, but can also act as a reservoir for pests and pathogens of potato.

A new analysis of soil temperature over a 25-year period reveals that the risk of volunteer potato growth is high in most areas of Michigan (Fig. 1). The probability of volunteer emergence is determined by the amount and depth of tubers remaining in the soil after harvest. With sufficient cold exposure, the tuber will not survive the winter, and tubers closer to the surface have more cold exposure than those deeper in the soil.

For the potatoes closer to the surface, there is a model developed by researchers at Michigan State University that considers the variation in soil temperature by depth. That model was used to estimate the risk of volunteer survival using soil temperatures at 2- and 4- inches, and the number of hours when soil temperature is under 27° F between Nov. 1 and March 31, referred to here as “frost hours”.

If a tuber is below 4 inches of soil, it is unlikely to be exposed to temperatures below the viability threshold of 50 frost-hours at or below 27° F. Over the last 25 years, there were several years when the entire state was at the highest risk value, meaning that potatoes survived most winters in the soil during the 2000-2024 period (Fig. 1A). Low potato volunteer risk seems to only occur in some years around the eastern side of the state in the Thumb, in the northern part of the lower peninsula and parts of the Upper Peninsula (Fig. 1B).

There were no years with low risk in the potato-growing areas of Montcalm and St. Joseph counties over the past 25 years. Significant changes were not seen in risk over time, with 2014-2015 being the only consecutive low-risk years during this time period. Overall, risk is slightly lower around the Saginaw Bay area, and the Southwestern edge of the Upper Peninsula, and uniformly high in Southern Michigan.

If the winter is warm enough to leave all potential volunteer tubers viable, there are chemical control methods, but they are not always reliable, and there is limited research in focal crops outside of field corn. Even if the above-ground portion is killed off, the parent tuber can produce new growth, and they often emerge too late to spray without damaging the crop plant they are intermixed with. Volunteer potatoes are an issue in crops grown in rotation with potatoes, including corn, winter wheat, soybean, dry beans, alfalfa, and sugar beet.

Volunteer potatoes are difficult to control but there is ongoing herbicide research to explore the efficacy of available options.

As our data analysis from the last 25 years indicates, waiting for a deep freeze to solve the volunteer potato problem is increasingly hopeless, and we need to invest into research to provide new solutions. Volunteer potatoes not only cause weed issues in rotational crops but they harbor pests such as insects and diseases that can infect new potato crops. Therefore, the impact of warming winters is more complicated and warrants the need for research that considers how these changing climate norms impact potato production.

Written by Abigail Cohen and Zsofia Pszendrei

Abigail Cohen, MSU, [email protected], is a Research Associate in the Department of Entomology, focused on analyzing and forecasting patterns in potato pests. Zsofia Szendrei, [email protected], is the Vegetable Entomologist at MSU.